Carbon-13 is a naturally occurring isotope of carbon, with 6 protons and 7 neutrons. It makes up about 1.1% of all natural carbon on Earth.
Stable Isotope: An isotope that has not observed to decay into other elements.
Natural Abundance: The measure of the average amount of a given isotope naturally occurring on Earth.
Neutron Number: The number of neutrons in an atomic nucleus.